Cooler, wetter weather: Thanks, corn and beans … I think

About now, farmers in Iowa and across the Corn Belt get itchy. As the weather warms, they start tuning plows and planters, preparing to put another crop of corn and soybeans in the ground.

Within months, the rural Midwest will largely be a sea of towering stalks filling out ears and squat bean plants putting on pods.

But this sea of biomass has unforeseen effects on Midwestern climate, a study from the Massachusetts Institute of Technology suggests – and, I would argue, contributes to global climate change.

When you’re driving past those carpeted fields this summer, you can thank them for countering higher temperatures driven by greenhouse gas emissions, but curse them for more frequent drenching, violent thunderstorms and tornadoes.

It stands to reason that agriculture – which has never been more intense or widespread in human history – is doing something to our weather. But there are bigger questions about its impact.The study is detailed in a recent story in the journal Science, from which I’m quoting. The study itself was published in the journal Geophysical Research Letters.

The researchers tackled a conundrum: While the world as a whole has warmed on average, summer temperatures have fallen by as much as a degree Celsius over the vast Midwestern expanse known as the Corn Belt. Rainfall over the region, meanwhile, has spiked by as much as 35 percent – the highest increase in the world, Science says.

The MIT team wanted to know whether crops could be the culprits. They ran computer simulations of five scenarios for 30-year periods, from 1982 to 2001.

Control models that didn’t include high levels of intense agriculture showed no changes in temperature or rainfall. In contrast, 62 percent of the simulations that included dense cropping produced temperature and rainfall changes that matched real-world conditionsof cooler, rainier weather.

The MIT group then compared its results to global simulations from the World Climate Research Programme (WCRP), an international organization that coordinates global climate research. Those models include the effects of increasing levels of greenhouse gases, like carbon dioxide and methane, that contribute to global warming. They don’t take agricultural land use into account.

The WCRP model said temperatures in the region should have increased only slightly and rainfall should have gone up just 4 percent – far short of reality. Science reports that other climate simulations that included variations in sea surface temperature also didn’t match the real-world data:

Those simulations matched historical data until 1970; after that, the simulations predicted temperatures to keep increasing, rather than decreasing as they did in reality. This is a strong indication that agriculture, and not changing sea surface temperature, caused the regional changes in climate during the last third of the 20th century, the researchers say.

That 1970 cut-off intrigued me. It was about that time the U.S. government stopped paying farmers not to plant acres, a strategy designed to limit production and support commodity prices. Instead, the government pushed producers to plant widely and export grain.

U.S. Department of Agriculture figures bear this out: Corn acres planted have risen steadily (with a dip in the mid-1980s) since the early 1970s. Farmers planted around 90 million acres in 2017, up by close to 35 percent over 1970’s roughly 67 million acres. Acres planted to soybean, meanwhile, have more than doubled, from 43 million in 1970 to 90 million in 2017.

And of course, farmers are planting more seeds per acre of corn and soybeans. That seed has been bred and engineered to produce leaps and bounds more from each plant.

As the crops grow, they release water vapor as part of photosynthesis. That water goes into the atmosphere and eventually falls as rain, the researchers postulate.

That got me wondering: Could the blankets of corn, soybeans and other crops across the Midwest contribute to the recurrent spring flooding Iowa has seen in recent years?

Probably not, says the study’s lead author, Ross Alter, an MIT postdoctoral associate who now is a meteorologist with the U.S. Army Corps of Engineers in New Hampshire: the timing is off.

“The greatest transfer of water from corn and soybeans to the atmosphere would occur during the summer (June to August), when crop growth is most intense,” Alter told me in an email. Crops haven’t grown enough in the spring to have that kind of impact.

Gene Takle, an Iowa State University professor and probably the state’s leading climate change expert, agreed. Our heavy spring rains are caused by increased moisture coming from a warmer Gulf of Mexico (a consequence of greenhouse-gas-caused climate change), he wrote in an email. Data going back to 1951 show increasing moisture flowing from the south into the Midwest, raising the average dew point temperature.

During summer, transpiring plants “likely contribute more recycled moisture that fuels our large” drenching thunderstorms, Takle wrote. “The magnitude will vary from year to year but may equal the impact of climate change.”

I asked Alter about this. “There is a large body of existing literature that supports the notion that humans can alter climate patterns, especially through greenhouse gas emissions,” he wrote. His team’s paper builds on a subset of these studies indicating that humans “can also impact regional climate through land-use changes.”

Takle’s take was similar: “There are many examples of land use changes having a major impact on regional climate,” often leading to drier conditions. “Trends toward more precipitation” are less likely, “but the Midwest is one such area.”

To me, these studies provide a clear answer: There’s no doubt human activity can alter regional climate. (Science notes that, like the Midwest, temperatures in areas of China engaged in intensive agricultural also run counter to warmer global temperatures.) And these regional changes, taken together, are part of the picture of human-caused global climate change.